David Hull stands in his driveway near ruts in the hard-packed sand that he recently filled into a sinkhole that appeared on his property in Reno, Texas. / Michael Mulvey for USA TODAY

by Rick Jervis, USA TODAY

by Rick Jervis, USA TODAY

RENO, Texas - The first time the earth shook their home, David and Meredith Hull thought it was a propane tank exploding outside, an odd but rare phenomenon.

Then it happened again. And again and again - more than 30 earthquakes since early November. One tremor tossed David Hull against the refrigerator and Meredith atop the stove.

"It felt like something was under the house literally lifting it up and slamming it back down on its foundation," said David Hull, 60, a retired sheriff's deputy. "The whole house was shaking."

The Hulls are one of dozens of families here and in nearby Azle, about 17 miles northwest of Fort Worth, who say they've been hit with a rash of earthquakes since November. Residents and city leaders point to area oil and gas disposal wells as likely culprits.

The wells dispose of wastewater used in hydraulic fracturing - or fracking - for natural gas in the nearby Barnett Shale.

Energy companies deny a direct link between the earthquakes and the wells, citing a lack of evidence. The vast majority of the 35,000 disposal wells throughout Texas have reported no seismic activity, said Bill Stevens of the Texas Alliance of Energy Producers. Still, the alliance has created a task force to look into the phenomenon.

"We feel comfortable that there is not a crisis," he said. "But we're also dedicated to pursuing it to the end."

The angst in North Texas has a familiar ring to it. Earthquakes in Colorado, Oklahoma, Ohio and Arkansas the past few years have all been tied to wastewater injection wells. New research presented at the Seismological Society of America annual meeting last month showed that disposal wells may be changing stress on existing faults and inducing earthquakes.

While Reno and Azle residents have only reported around 30 earthquakes to the U.S. Geological Survey, area seismologists have recorded more than 300 quakes in the area since December - many too small for human detection - all clustered around area injection wells.

The earthquakes have been relatively small, less than a magnitude 4.0 - not big enough to cause major damage but alarming enough to spur state leaders into action. In March, the Texas Railroad Commission, which oversees the oil and gas industry, took the rare step of hiring a seismologist to study the matter.

In early May, Texas legislators convened a House Subcommittee on Seismic Activity to gather testimony from affected residents and experts. A team of seismologists from Southern Methodist University in Dallas is also investigating a possible link between earthquakes and the wells.

The quakes have cracked ceilings in homes and left gaping sinkholes in fields, said Reno Mayor Lynda Stokes, who, along with Azle Mayor Alan Brundrett, testified at the subcommittee hearing last month.

"We just want it to quit," said Stokes, whose house has been repeatedly shaken by tremors, sometimes twice a day. "What they're doing is not safe."

Earthquakes in Texas are historically rare. But in 2008, a rash of earthquakes began in the Dallas-Fort Worth area near where energy companies were fracking for natural gas, said Cliff Frohlich, associate director of the Institute for Geophysics at the University of Texas, who studied the tremors. Frohlich and others measured more than 60 quakes over a two-year period, all near injection wells.

Reno and Azle, as well as Dallas, sit on small fault lines several miles underground, Frohlich said. Wastewater blasted into disposal wells at high speeds could potentially disturb those otherwise dormant faults, causing them to slip and induce earthquakes, he said.

However, faults also exist in South Texas, an area currently experiencing a fracking boom with thousands of wastewater wells, yet is reporting little to no seismic activity, Frohlich said. Also, the Bakken Shale in North Dakota is in the midst of an oil boom thanks to hydraulic fracturing - yet no earthquakes.

In Reno, the horses often signal a looming earthquake. They'll start to neigh and gallop from one end of their pen to the other a few seconds before a tremor hits, David Hull said.

The quakes only last a few seconds but have splintered a window, cracked an outside wall and fractured all three toilets, he said. His insurance company won't cover the damages because they say the earthquakes are man-made, David Hull said.

It's been relatively quiet since January, with just a few small tremors last month. But living with earthquakes was not something he considered when retiring to his 11-acre ranch.

"An earthquake in this part of the country was the furthest thing from my mind," said David Hull, who lived most of his adult life in North Texas without ever feeling a tremor. "But they're here."